The present invention relates to simulation of Resistive Random-Access Memory (RRAM) or other two-terminal resistive devices with hysteresis. Such devices are also sometimes termed memristors.
RRAM is a type of resistive memory that has generated significant interest as a potential candidate for ultra-high density non-volatile information storage. Fabrication costs of RRAM designs are substantial, so considerable savings can be realized by optimizing design by modeling RRAM in a simulated environment. As RRAM technology becomes available to circuit designers, there will be an increasing need for accurate modeling and simulation tools.
A working principle of RRAM devices is the formation of a conductive path governed by a filamentary process. In an exemplary device, in the presence of an applied electric field, silver ions migrate through an amorphous silicon solid electrolyte to form a conductive filament along which electrons can travel. As current passes through the device, Joule heating raises the device temperature, which affects filament growth. Filament growth exists in at least two different forms, including extension of length between electrodes and an increase in width, both of which have an effect on the electrical properties of an RRAM cell.
Two-terminal RRAM has several unusual properties. For example, under the application of a bias voltage, the activation energy of a silver ion varies with distance as it moves from a source electrode to a destination electrode, and the amplitude of subsequent energy peaks decreases with respect to distance as the ion passes through imperfections in a semiconductor matrix. When a two-terminal RRAM cell is coupled to a resistor in series, variance of the resistance value of the resistor can affect the final resistance of the RRAM cell, thereby allowing the cell to retain additional data.
Current simulation models typically utilize a fixed threshold voltage to activate an RRAM write cycle. Such models assume that a single set voltage will result in changing a resistance value from an ON state to an OFF state, or vice versa. However, the threshold voltage is dynamic in an actual two-terminal RRAM device. Using a fixed voltage fails to capture some of its unique properties, and otherwise results in an inaccurate simulation.
Current simulation models typically utilize a fixed switching time to activate an RRAM write cycle. However, the switching time is dynamic in an actual two-terminal RRAM device and varies with respect to voltage. Using a fixed switching time fails to capture some of its unique properties, and otherwise results in an inaccurate simulation.
There is a need for an accurate simulation model that accurately models the dynamic characteristics of a two-terminal RRAM cell. Although equations have been established to represent some aspects of memory cell performance, existing simulation models are not capable of simultaneously accounting for the variables of ion growth and current in the greater context of a circuit. An RRAM simulation method and system capable of resolving the dynamic relationship between state variables and properties in an RRAM cell would aid the task of incorporating these important new technologies into circuit designs, and help bring the use of RRAM devices closer to realization.